This invention relates to transmission systems and more particularly, to digital transmission systems using orthogonal frequency division multiplexing (OFDM). This invention also relates to a transmitter and receiver adapted to such a system.
Recently there has been an increasing interest in providing high data-rate services such as video-conferencing, multi-media Internet access and wide area network over wide-band wireless channels. Wideband wireless channels available in the PCS band (2 GHz) have been envisioned to be used by mobile (high Doppler) and stationary (low Doppler) units in a variety of delay spread profiles. This is a challenging task, given the limited link power budget of mobile units and the severity of wireless environment, and calls for the development of novel robust bandwidth efficient techniques that work reliably at low SNRs.
The OFDM transmission system is a variation of the multiple carrier modulation system. FIG. 1 depicts a conventional OFDM system. A frame of bits is applied to serial-to-parallel converter 10 where it is divided into n multi-bit complex symbols c1 through ca and delivered simultaneously to inverse Fourier transformer 20. Discrete Fourier transformer 20 develops a time signal that corresponds to a plurality of individual carrier signals which are amplitude modulated by symbols c1 through ca. This signal is modulated up to the desired band by amplitude modulator 30, and transmitted.
At the receiver, the received signal is modulated down to baseband by converter 40, and applied to discrete Fourier transformer 50. Transformer 50 performs the inverse operation of Fourier transformer 20 and, thereby (in the absence of corruption stemming from noise), recovers symbols c1 through ca. A parallel to serial converter 60 reconstitutes the serial flow of symbols c1 through ca and converts the symbols to individual bits.
Separately, space-time coding was recently introduced for narrowband wireless channels U.S. patent applications having the Ser. Nos. 09/063,765, 08/847,635 and 08/114,838, are examples of such systems. These systems encode the signals and employ both time and space diversity to send signals and to efficiently recover them at a receiver. That is, a set of symbols is encoded, for example in various permutations, and the encoded signals are transmitted over a number of antennas (providing the space diversity) and a number of time slots (providing time diversity). Of course, that requires use of a number of time slots for each set of symbols. For channels with slowly varying channel characteristics, where it can be assumed that the characteristics do not change from frame to frame, the decoding process can be simplified.
An advance in the art is achieved by employing the principles of Orthogonal Frequency Division Multiplexing (OFDM) in combination with a plurality of transmitting antennas. That is, an arrangement is created where a transmitter includes a plurality of antennas that are transmitting simultaneously over the same frequency subbands, and the symbols that are transmitted over each subband, in any given time slot, over the different antennas are encoded to provide diversity. The principles of trellis coding, space-time coding, or any other diversity-producing coding can be applied in this arrangement. Illustratively, each given subband being transmitted out of the plurality of transmitting antenna can be treated as belonging to a space-time encoded arrangement and the symbols transmitted over the given subband can then be encoded in block of pxc3x97n symbols, where n is the number of transmitting antennas, and p is the number of time slots over which the block of symbols is transmitted.